A nanowire refers to a wire having a diameter from about 1 nm to about 1,000 nm. Implied in the use of the term “nanowire” is the assumption that the length of the nanowire is substantially greater than the diameter of the nanowire. Nanowires having a diameter at a low end of the range, i.e., a diameter from about 1 nm to about 100 nm, exhibit quantum mechanical properties, and are also called “quantum wires.” A nanowire may comprise a metallic material, a semiconductor material, or an insulator material. Both organic nanowires, e.g., deoxyribonucleic acid (DNA), and inorganic nanowires are known in the art.
The ratio of the length of a nanowire to the diameter of the nanowire is referred to as an aspect ratio. Typical aspect ratios for nanowires range from about 10 to about 1,000,000. As such, one dimension, i.e., the length, of a nanowire may be considered to be virtually infinite for practical purposes, while the diameter of the nanowire determines predominant physical and chemical characteristics of the nanowire. Quantum mechanical properties of nanowires due to the limited dimension of the nanowire in the plane perpendicular to the length of the nanowire have been observed in many types of nanowires.
Devices employing semiconductor nanowires have been proposed in the art. One of the challenges in the manufacture of the semiconductor devices employing semiconductor nanowires has been the difficulty in forming doped semiconductor nanowires. Particularly, controlled doping of semiconductor nanowires comprising silicon or germanium presents a significant challenge.
Prior art publications propose doping of semiconductor nanowires by a dopant gas such as phosphene, diborane, etc. with a reactant gas such as silane or germane during the growth of the nanowire. One such example is U.S. Pat. No. 6,962,823 to Empedocles et al., which describes in-situ doping of isolated bands in a semiconductor nanowire by changing of dopant level during growth of the semiconductor nanowire. Application of this approach to form a silicon nanowire or a geranium nanowire faces severe problems that hamper its usefulness. In particular, a relatively low temperature range from about 350° C. to about 450° C. is required to grow certain types of semiconductor nanowires, such as silicon nanowires and germanium nanowires, with diameters less than 30 nm. At an elevated temperature above 500° C., such semiconductor nanowires grow laterally.
Unfortunately, a relatively high temperature above 500° C. is required to successfully incorporate dopant atoms at a significant level, i.e., at a concentration of about 1.0×1017/cm3 or above, into the semiconductor nanowire during growth. However, at such a temperature, the semiconductor nanowire also grows laterally since the elevated temperature enables pyrolysis of reactants on the exposed surfaces of the semiconductor nanowire. Suppression of pyrolysis of reactants on sidewall surfaces of semiconductor nanowires and doping of the sidewall surfaces of the semiconductor wires are mutually incompatible in many semiconductor nanowires. In other words, doping of the sidewall surfaces of the semiconductor wires necessarily causes pyrolysis of reactants on the sidewall surfaces the semiconductor nanowires.
This situation is illustrated in FIGS. 1 and 2. In FIG. 1, a catalyst particle 20, such as a gold particle, is placed on a substrate 10, which may comprise an insulator material or a semiconductor material. In a temperature range from about 350° C. to about 450° C., a semiconductor nanowire 30 is grown as reactants containing a semiconductor material, e.g., SiH4, Si2H6, GeH4, etc., are supplied into a reactor, such as a low pressure chemical vapor deposition (LPCVD) chamber or an ultra-high vacuum chemical vapor deposition (UHVCVD) chamber. The catalyst particle 20 forms a thin moving eutectic region (not shown) at the interface between the catalyst particle 20 and the top region of the semiconductor nanowire 30, and travels at the top of the semiconductor nanowire throughout the growth process. The diameter, or the lateral dimensions, of the semiconductor nanowire 30 is substantially the same as the lateral dimensions of the catalyst particle 20. At this temperature range, dopant incorporation into the semiconductor nanowire 30 is negligible. Thus, even if a high partial pressure is maintained for dopant gases in a reactor, dopant incorporation into the semiconductor nanowire 30 is insignificant, and the semiconductor nanowire 30 remains essentially undoped.
Referring to FIG. 2, to force incorporation of dopants, the temperature of the reactor must be increased above 500° C. in the case of some semiconductor wires such as a silicon nanowire and a germanium wire. In this case, however, pyrolysis occurs on the sidewalls of a doped semiconductor nanowire 40. While dopants are incorporated into the doped semiconductor nanowire 40 during the growth, lateral growth of the doped semiconductor wire 40 is unavoidable. Further, a bottom portion of the doped semiconductor wire 40, being exposed to the reactants for a longer time, has a larger diameter than a top portion of the doped semiconductor wire 40, which is exposed to the reactants for a shorted period of time, and has lateral dimensions close to lateral dimensions of the catalyst particle 20. Thus, the doped semiconductor wire 40 has a taper in the diameter, i.e., a constant diameter for the doped semiconductor wire 40 cannot be achieved.
An alternate approach to forming doped semiconductor wires is to implant dopants into nanowires. However, ion implantation tends to be an inherently violent process due to the high energy of the implanted ions, and causes amorphization of the nanowire crystalline structure and sputtering of the nanowire itself. Thus, the ion implantation tends to cause wire deformation and/or wire breakage during the ion implantation process. Subsequent anneals are required to recrystallize the nanowire, which may not be wholly effective if the structural damage is extensive.
While prior art publications, such as U.S. Pat. No. 7,105,428 to Pan et al., and U.S. Pat. No. 7,211,464 to Lieber et al., disclose useful applications of doped semiconductor nanowires, methods of forming such useful structures still need to be provided to enable such structures.
Thus, there exists a need for methods of forming a semiconductor wire having constant lateral dimensions and a significant level of doping, i.e., at a concentration of about 1.0×1017/cm3 or above.
Further, there exists a need for methods of forming a doped semiconductor wire having constant lateral dimensions, such as a diameter, of less than 30 nm.
In this disclosure we describe a simple method for controllably incorporating dopant atoms into semiconductor nanowires. This process separates the initial growth of the nanowire from the doping process, thereby allowing the growth of the nanowire to be decoupled from dopant incorporation into the nanowire crystalline structure.